OAKLAND -- The Oakland Museum of California is offering a $12,000 reward for a Gold Rush-era quartz and gold encrusted jewel box stolen from the permanent collection during the second heist in as many months.

The thief took the historic "Comstock Jewel Casket," which is covered in California gold, from a museum exhibit in the history wing Monday morning while the museum was closed.

A second artifact also was taken, but officials wouldn't release information about what the item was or how much both pieces are worth. They are particularly worried the burglar will try to melt the irreplaceable box made from gold-bearing moss quartz that hasn't been mined in California since the 19th century and which was stolen and returned once before to the museum in 1978.

"This is a theft of our history and the heritage of our children," Mayor Jean Quan said Wednesday.

On Monday, the thief apparently forced his way into the second floor exhibit through an emergency exit door just a few feet from where the 3-pound jewel box was encased in a Plexiglas display wired to an alarm.

A security guard heard the alarm and could see the intruder on a screen in another section of the compound that stretches for a city block along Oak Street.

The guard called Oakland police, but the thief had already left by the time officers arrived.

The description of the man was vague except that he was wearing a white mask, "which is relatively unique," said Lt. Oliver Cunningham Wednesday at the museum.

Cunningham said investigators believe the thief was the same man who broke into the same exhibit about 11:30 p.m. on Monday, Nov. 12.

Gold nuggets and other, undisclosed artifacts, some more than 200 years old, were stolen during that break-in.

The museum increased security after that burglary.

The heightened security worked, "but obviously we would like the security measures to be better or this wouldn't have happened," museum Executive Director Lori Fogarty said.

Other than these two burglaries, Fogarty said she couldn't remember another break-in since she arrived at the museum seven years ago.

The museum, however, has experienced several important changes recently.

The Oakland Museum of California Foundation took over running the day-to-day operations in July 2011 and recently finished extensive renovations.

The museum collection belongs to the city of Oakland, and this is the second time the coveted casket, made about 1875, has escaped the museum's control. The first time was after hours on March 1978 and was the first forced-entry theft in the institution's more than 40-year history.

That year, a thief forced his way into the room through a door from the garden and stole the Comstock Casket, about the size of a small shoe box, made by an artisan San Francisco goldsmith, A. Andrews, as a wedding anniversary gift in the 19th century. He had decorated the inside with scenes of trains crossing the plain, Indians on horseback and buffalo herds, and the box rested on four feet formed of miniature representations of Minerva, a symbol of California.

An anonymous donor purchased the box for $11,400 and gave it to the museum in 1969 when it opened. An ivory Eagle valued at $10,000 also was stolen but was recovered when police arrested the thief, William E. Murray.

However, seven years passed before the Comstock Casket finally landed in the hands of the Butterfield and Butterfield Auctioneers and Art Appraisers in San Francisco.

The casket was valued at $100,000, said Michael Weller, owner of Argentum Antiques in San Francisco, shortly after he helped reunite the Comstock Casket with the museum in 1985.

The casket hasn't been appraised since then, Fogarty said Wednesday, although it is insured.

"It is less about monetary value than the history the artifact embodies," Fogarty said. "It is a critical piece."

Police are asking anyone with information about the box to call 510-238-3951.